Integral image elements which use a lenticular lens sheet or a fly's eye lens sheet, and a three-dimensional integral image aligned with the sheet, so that a user can view the three-dimensional image without any special glasses or other equipment, are known. Such imaging elements and their construction, are described in "Three-Dimensional Imaging Techniques" by Takanori Okoshi, Academic Press, Inc., New York, 1976. Integral image elements having a lenticular lens sheet (that is, a sheet with a plurality of adjacent, parallel, elongated, and partially cylindrical lenses) are also described in the following Unites States patents: U.S. Pat. No. 5,391,254; U.S. Pat. No. 5,424,533; U.S. Pat. No. 5,241,608; U.S. Pat. No. 5,455,689; U.S. Pat. No. 5,276,478; U.S. Pat. No. 5,391,254; U.S. Pat. No. 5,424,533 and others; as well as allowed U.S. patent application Ser. No. 07/931,744. Integral image elements with lenticular lens sheets use interlaced vertical image slices which, in the case of a three-dimensional integral image, are aligned with the lenticules so that a three-dimensional image is viewable when the lenticules are vertically oriented with respect to a viewer's eyes. Similar integral image elements, such as described in U.S. Pat. No. 3,268,238 and U.S. Pat. No. 3,538,632, can be used to convey a number of individual two-dimensional scenes (such as unrelated scenes or a sequence of scenes depicting motion) rather than one or more three-dimensional images.
Integral image elements using reflective layers behind the integral image to enhance viewing of the integral image by reflected light, are also described in U.S. Pat. No. 3,751,258, U.S. Pat. No. 2,500,511, U.S. Pat. No. 2,039,648, U.S. Pat. No. 1,918,705 and GB 492,186.
In a typical method of assembling a lenticular type of integral image element, an original negative is exposed from stored digitized data of a composite lenticular image on a film writer. A suitable film writer is the Symbolic Sciences International Fire 1000 and the LVT Model 1620B, available from Light Valve Technology, a subsidiary of Eastman Kodak Company, Rochester, N.Y. A suitable negative exposure technique is disclosed in U.S. Pat. No. 5,276,478. After photographic processing, the negative is printed, typically by a projection enlarger, onto a suitable film- or paper-based photographic print stock. After processing, the lenticular composite print is coated with adhesive, aligned with a lenticular lens sheet, and pressed against the lens sheet to permanently adhere to it in proper registration with the printed lenticular composite image. However, it is also known to write the lenticular image directly onto a back side of a lenticular lens sheet which is coated with a suitable receiving layer, such as disclosed in U.S. Pat. No. 5,349,419 and U.S. Pat. No. 5,279,912. Furthermore, such "writing" of the lenticular image can be temporary, as in a display produced on a CRT or Liquid Crystal Display ("LCD") screen immediately adjacent the back side. There are indications that lenticular imagery may increasingly be applied to LCD screens and the like to make effective 3D video a commercial reality.
Typically, in manufacturing a lenticular lens sheet a hot plastic melt composition is contacted with a hollow drum carrying a series of adjacent, concave, grooves of semi-circular transverse cross-section, along the length of its cylindrical surface. The drum is heated to a controlled temperature by water flow therethrough. Hot plastic composition cools on the rotating drum surface to form a continuous lenticular lens sheet with the lens elements extending along the direction of the continuous lens sheet. Such techniques are well known in the art of forming lenticular lens sheets. However, in other methods the grooves can be along the length of the cylinder, so that the final lenticules formed in the continuous lens sheet are oriented across the lens sheet. Since each of the lenses of a lenticular lens sheet is dedicated to one or more corresponding sets of interleaved image lines, it is important during manufacture of a continuous lens sheet or multiple individual lens sheets, that the lenticular lens sheet be of well controlled quality. Variations in lens pitch (which for the usual case of immediately adjacent lenses, is equal to the lens width) will result in inaccurate matching of individual lenses with corresponding line sets and consequently poor images when viewed. Similarly, the lens sheet thickness should remain relatively constant so that the image lines will be positioned at the individual lens focal points (whether this is as a result of being positioned directly on the back side of the lens sheet or being separated therefrom). Other lens characteristics such as refractive index of the plastic material, and lens shape (affecting image display geometry and image ghosting), should also preferably remain relatively constant. However, one or more of these parameters may vary during manufacture of a continuous lens sheet as a result of changes in the temperature of the roller or the composition being fed in, flow rates of the hot plastic composition or variations in roller speed.
The ability to produce high quality lenticular lens sheets has always been a definite factor in the commercial feasibility of all types of lenticular imagery. In current techniques, the image quality of lenticular optics is typically postevaluated with a microscope, the thickness measured with micrometers, the pitch measured with optical comparators, the surface profiles traced with profilometers, and index variations can be assessed with immersion refractometry and interferometry. However, none of these techniques has been successful in evaluating the optical performance in a way directly analogous to the eventual assembly performance with the printed image. Additionally, none of these techniques readily lend themselves to high volume production of a continuous lens sheet or individual lens sheets, where the lens sheet characteristics need to be measured rapidly so that incorrect conditions which result in poor quality lens sheets can be corrected before further incorrect lens sheet material is formed. The control of thickness in an extrusion process can be monitored with scanning atomic radiation devices and feedback signals can be used to vary certain extrusion parameters affecting thickness. However, a question always remains of whether the nominal thickness being maintained is the correct thickness for lenticular performance in the assembly application (in which an image is aligned with the lens sheet). That is, the measurement itself is not directly related to the manner of use of the lens sheet.
It would be desirable then, to have a means for assessing the quality of a lenticular lens sheet, in particular one or more of the parameters described above, which is rapid to allow its application to high volume production of lens sheet and which provides indications of quality which are related to the manner of use of the lens sheet.